1. Technical Field
The present invention relates to a tubular linear motor that exerts a rectilinear motion to a driving target based on an electromagnetic force.
2. Description of Related Arts
In general, a tubular linear motor includes a magnetic exciter in which a plurality of permanent magnets is arranged in series such that different magnetic poles face each other, and an armature having a coil, arranged to surround the magnetic exciter and slidable along an axial direction of the magnetic exciter.
In the tubular linear motor, an electric current flows through the coil of the armature to intersect a magnetic flux generated by the permanent magnet of the magnetic exciter, so that a driving force in an axial direction is generated in the coil based on the electromagnetic induction effect to move the armature.
As a technique relating to the armature of the linear motor, there are disclosed a linear synchronous motor and a linear actuator, in which a thrust force ripple and a cogging force are reduced by setting a magnetic gap (distance from a tooth leading edge to the magnet surface) of the teeth positioned in both ends of an axial direction to be narrow and regularizing a magnetic flux density between a plurality of teeth (for example, refer to WO 2007/040009 A).
As a technique relating to the magnetic exciter of the linear motor, there is disclosed a linear motor in which a stator unit is configured such that a center shaft made of a nonmagnetic material is inserted into the permanent magnets having a tubular shape, and the neighboring permanent magnets closely abut on each other by strongly clamping the permanent magnets from both ends (for example, refer to JP 10-313566 A).
However, in the technique disclosed in WO 2007/040009 A, a magnetic flux distribution is different between both end teeth of an axial direction and a center tooth, so that an edge effect is enhanced due to the narrow magnetic gap. Therefore, if a core deviation (eccentric core) is generated in the center shaft, a thrust force ripple and a cogging force unfortunately increase due to influence of the magnetic attractive force.
In the technique disclosed in JP 10-313566 A, the neighboring permanent magnets closely abut on each other. Therefore, the magnetic flux can reach farther, whereas an average magnetic flux is low, and there is a limitation to increase the thrust force.
In addition, since the magnets are arranged to repulsively interact with each other, it is difficult to manufacture the magnetic exciter by overcoming the strong magnetic repulsive force.
Furthermore, the permanent magnets are arranged to closely abut on each other around the center shaft, and the soft magnetic body is not provided between the permanent magnets. Therefore, consumption of the permanent magnetic material is large, and the manufacturing cost of the linear motor increases.